Mold is a common allergen that can grow in many locations inside or outside a dwelling. It can also be found thriving inside building cavities, between walls. Mold is a very common indoor contaminant, and a common cause of illness. In a study of several hundred Tennessee homes in the United States, 49% had airborne concentrations of fungi greater than the concentration considered by some allergists to be acceptable to sensitive individuals. Only a few dozen of the thousands of different types of mold are commonly found in dwellings for humans.
Molds reproduce by releasing spores into the air. The spores are extremely small, about 1 micron or about 0.00004 inches. Mold counts are often 1,000 times higher than pollen counts. Although tiny parts of the parent mold colony can break off and be inhaled, usually, inhaled microscopic spores are the source of health problems. A person's allergic response is a biological reaction to the protein in mold, so the reaction can occur whether the inhaled spores are dead or alive. A thriving mold colony often releases various gases, including volatile organic compounds, that are also a problem for sensitive individuals.
Different species of mold have different health effects ranging from mild symptoms to death. In Cleveland, Ohio, in 1993, there were at least 34 cases of infants with bleeding in the lungs. The condition was related to the mold Stachybotrys atra. At least 10 deaths were recorded. Some species of the mold Aspergillus can infect the entire body of a person, causing lung damage or other serious illnesses. Histoplasma capsulatum can affect the lungs, but can also be systemic. A mold colony can use any organic material for food, and can even derive nutrition from a layer of dust on non-organic surfaces.
Mold requires five ingredients to thrive: food, air, a surface to grow upon, suitable temperature, and moisture. In an occupied building, little can be done to eliminate the first four conditions. In these instances, only the manipulation of moisture can be used to eliminate a mold colony or to prevent a new colony from forming. Another factor in mold growth is a change in barometric pressure. Sporalation can be encouraged by a reduction in the barometric pressure. In nature, a storm front and the accompanying higher humidity levels and wet weather are normally preceded by a reduction in barometric pressure.
Mold growth is related to relative humidity. Relative humidity levels below about 70% will not support excessive mold growth. However, indicated relative humidity levels below 70% do not ensure safety. Although a house may have 60% relative humidity, microclimates of higher relative humidity may exist throughout the house, especially near cooler surfaces. This is because cold air cannot support as much water moisture as warm air. Thus, for a given amount of water vapor in the air, the cooler air will have a higher relative humidity.
For example, assume the air in a house has a relative humidity of 60% at 21° C. (70° F.). The air outside the house is 10° C. (50° F.), and the air between the outside wall and the inner drywall is at 16° C. (60° F.). Furthermore, the air in the house and the air between the walls can circulate, which is very common. In this case, the 16° C. air within the wall cavity will have a relative humidity of 70%, and may support excessive mold growth.